If you’re looking for a SIM only solution, check out giffgaff’s £12 goodybag. It contains 250 minutes, unlimited texts and unlimited data. Nice.
No specific Fair Use Policy, though traffic management may be enforced in extreme circumstances. More info here.
giffgaff operates on the O2 network.
Tethering not permitted.
(Thanks to Tommyboy02 at HUKD)